Apparatus and method for in-situ endpoint detection for chemical mechanical polishing operations

ABSTRACT

An apparatus for chemical mechanical polishing (CMP) of a wafer has a rotatable platen to hold a polishing pad, a polishing head for holding the wafer against the polishing pad, an optical monitoring system and a position sensor. The platen has a hole therein, the optical monitoring system includes a light source to direct a light beam through the aperture toward the wafer from a side of the wafer contacting the polishing pad and a detector to receive reflections of the light beam from the wafer, and the position sensor senses when the hole is adjacent the wafer such that the light beam generated by the light source can pass through the hole and impinge on the wafer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of (and claims priorityunder 35 USC 120 to) pending U.S. application Ser. No. 11/099,789, filedApr. 5, 2005, which is a continuation of U.S. application Ser. No.09/399,310, filed Sep. 20, 1999, which is a continuation of U.S.application Ser. No. 08/979,015, filed Nov. 26, 1997, now abandoned,which is a file-wrapper-continuation of U.S. application Ser. No.08/413,982, filed Mar. 28, 1995, now abandoned. The disclosure of eachof the prior applications is considered part of (and is incorporated byreference in) the disclosure of this application.

BACKGROUND

1. Technical Field

This invention relates to semiconductor manufacture, and moreparticularly, to an apparatus and method for chemical mechanicalpolishing (CMP) and in-situ endpoint detection during the CMP process.

2. Background Art

In the process of fabricating modern semiconductor integrated circuits(ICs), it is necessary to form various material layers and structuresover previously formed layers and structures. However, the priorformations often leave the top surface topography of an in-process waferhighly irregular, with bumps, areas of unequal elevation, troughs,trenches, and/or other surface irregularities. These irregularitiescause problems when forming the next layer. For example, when printing aphotolithographic pattern having small geometries over previously formedlayers, a very shallow depth of focus is required. Accordingly, itbecomes essential to have a flat and planar surface, otherwise, someparts of the pattern will be in focus and other parts will not. In fact,surface variations on the order of less than 1000 Å over a 25×25 mm diewould be preferable. In addition, if the aforementioned irregularitiesare not leveled at each major processing step, the surface topography ofthe wafer can become even more irregular, causing further problems asthe layers stand up during further processing. Depending on the die typeand the size of the geometries involved, the aforementioned surfaceirregularities can lead to poor yield and device performance.Consequently, it is desirable to effect some type of planarization, orleveling, of the IC structures. In fact, most high density ICfabrication techniques make use of some method to form a planarizedwafer surface at critical points in the manufacturing process.

One method for achieving the aforementioned semiconductor waferplanarization or topography removal is the chemical mechanical polishing(CMP) process. In general, the chemical mechanical polishing (CMP)process involves holding and/or rotating the wafer against a rotatingpolishing platen under a controlled pressure. As shown in FIG. 1, atypical CMP apparatus 10 includes a polishing head 12 for holding thesemiconductor wafer 14 against the polishing platen 16. Theaforementioned polishing platen 16 is typically covered with a pad 18.This pad 18 typically has a backing layer 20 which interfaces with thesurface of the platen and a covering layer 22 which is used inconjunction with a chemical polishing slurry to polish the wafer 14.Although some pads 18 have only the covering layer 22, and no backinglayer. The covering layer 22 is usually either an open cell foamedpolyurethane (e.g. Rodel IC1000), or a sheet of polyurethane with agrooved surface (e.g. Rodel EX2000). The pad material is wetted with theaforementioned chemical polishing slurry containing both an abrasive andchemicals. One typical chemical slurry includes KOH (PotassiumHydroxide) and fumed-silica particles. The platen is usually rotatedabout its central axis 24. In addition, the polishing head is usuallyrotated about its central axis 26, and translated across the surface ofthe platen 16 via a translation arm 28. Although just one polishing headis shown in FIG. 1, CMP devices typically have more than one of theseheads spaced circumferentially around the polishing platen.

A particular problem encountered doing a CMP process is in thedetermination that a part has been planarized to a desired flatness orrelative thickness. In general, there is a need to detect when thedesired surface characteristics or planar condition has been reached.This has been accomplished in a variety of ways. Early on, it was notpossible to monitor the characteristics of the wafer during the CMPprocess. Typically, the wafer was removed from the CMP apparatus andexamined elsewhere. If the wafer did not meet the desiredspecifications, it had to be reloaded into the CMP apparatus andreprocessed. This was a time consuming and labor-intensive procedure.Alternately, the examination might have revealed that an excess amountof material had been removed, rendering the part unusable. There was,therefore, a need in the art for a device which could detect when thedesired surface characteristics or thickness had been achieved, in-situ,during the CMP process.

Several devices and methods have been developed for the in-situdetection of endpoints during the CMP process. For instance, devices andmethods that are associated with the use of ultrasonic sound waves, andwith the detection of changes in mechanical resistance, electricalimpedance, or wafer surface temperature, have been employed. Thesedevices and methods rely on determining the thickness of the wafer or alayer thereof, and establishing a process endpoint, by monitoring thechange in thickness. In the case where the surface layer of the wafer isbeing thinned, the change in thickness is used to determine when thesurface layer has the desired depth. And, in the case of planarizing apatterned wafer with an irregular surface, the endpoint is determined bymonitoring the change in thickness and knowing the approximate depth ofthe surface irregularities. When the change in thickness equals thedepth of the irregularities, the CMP process is terminated. Althoughthese devices and methods work reasonably well for the applications forwhich they were intended, there is still a need for systems whichprovide a more accurate determination of the endpoint.

SUMMARY

The present invention is directed to a novel apparatus and method forendpoint detection which can provide this improved accuracy. Theapparatus and method of the present invention employ interferometrictechniques for the in-situ determination of the thickness of materialremoved or planarity of a wafer surface, during the CMP process.

Specifically, the foregoing objective is attained by an apparatus andmethod of chemical mechanical polishing (CMP) employing a rotatablepolishing platen with an overlying polishing pad, a rotatable polishinghead for holding the wafer against the polishing pad, and an endpointdetector. The polishing pad has a backing layer which interfaces withthe platen and a covering layer which is wetted with a chemical slurryand interfaces with the wafer. The wafer is constructed of asemiconductor substrate underlying an oxide layer. And, the endpointdetector includes a laser interferometer capable of generating a laserbeam directed towards the wafer and detecting light reflected therefrom,and a window disposed adjacent to a hole formed through the platen. Thiswindow provides a pathway for the laser beam to impinge on the wafer, atleast during the time that the wafer overlies the window.

The window can take several forms. Among these are an insert mountedwithin the platen hole. This insert is made of a material which ishighly transmissive to the laser beam, such as quartz. In thisconfiguration of the window, an upper surface of the insert protrudesabove a surface of the platen and extends away from the platen adistance such that a gap is formed between the upper surface of theinsert and the wafer, whenever the wafer is held against the pad. Thisgap is preferably made as small as possible but without allowing theinsert to touch the wafer. Alternately window can take the form of aportion of the polishing pad from which the adjacent-backing layer hasbeen removed. This is possible because the polyurethane covering layeris at least partially transmissive to the laser beam. Finally, thewindow can take the form of a plug formed in the covering layer of thepad and having no backing layer. This plug is preferably made of apolyurethane material which is highly transmissive to the laser beam.

In one embodiment of the present invention, the hole through the platen,and the window, are circular in shape. In another, the hole and windoware arc-shaped. The arc-shaped window has a radius with an origincoincident to the center of rotation of the platen. Some embodiments ofthe invention also have a laser beam whose beam diameter that at itspoint of impingement on the wafer is significantly greater than thesmallest diameter possible for the wavelength employed.

The aforementioned CMP apparatus can also include a position sensor forsensing when the window is adjacent the wafer. This ensures that thelaser beam generated by the laser interferometer can pass unblockedthrough the window and impinge on the wafer. In a preferred embodimentof the invention, the sensor includes a flag attached along a portion ofthe periphery of the platen which extends radially outward therefrom. Inaddition, there is an optical interrupter-type sensor mounted to thechassis at the periphery of the platen. This sensor is capable ofproducing an optical beam which causes a signal to be generated for aslong as the optical beam is interrupted by the flag. Thus, the flag isattached to the periphery of the platen in a position such that theoptical beam is interrupted by the flag, whenever the laser beam can bemade to pass unblocked through the window and impinge on the wafer.

Further the laser interferometer includes a device for producing adetection signal whenever light reflected from the wafer is detected,and the position sensor includes an element for outputting a sensingsignal whenever the window is adjacent the wafer. This allows a dataacquisition device to sample the detection signal from the laserinterferometer for the duration of the sensing signal from the positionsensor. The data acquisition device then employs an element foroutputting a data signal representing the sampled detection signal. Thisdata acquisition device can also include an element for integrating thesampled detection signal from the laser interferometer over apredetermined period of time, such that the output is a data signalrepresenting the integrated samples of the detection signal. In caseswhere the aforementioned predetermined sample period cannot be obtainedduring only one revolution of the platen, an alternate method ofpiece-wise data acquisition can be employed. Specifically, the dataacquisition device can include elements for performing the method ofsampling the detection signal output from the laser interferometerduring each complete revolution of the platen for a sample time,integrating each sample of the detection signal over the sample time toproduce an integrated value corresponding to each sample, and storingeach integrated value. The data acquisition device then uses otherelements for computing a cumulative sample time after each completerevolution of the platen (where the cumulative sample time is thesummation of the sample times associated with each sample of thedetection signal), comparing the cumulative sample time to a desiredminimum sample time, and transferring the stored integrated values fromthe storing element to the element for calculating a summation thereof,whenever the cumulative sample time equals or exceeds the predeterminedminimum sample time. Accordingly, the aforementioned output is a datasignal representing a series of the integrated value summations from thesummation element.

The data signal output by the data acquisition device is cyclical due tothe interference between the portion of the laser beam reflected fromthe surface the oxide layer of the wafer and the portion reflected fromthe surface of the underlying wafer substrate, as the oxide layer isthinned during the CMP process. Accordingly, the endpoint in a CMPprocess to thin the oxide layer of a blank oxide wafer can be determinedusing additional apparatus elements for counting a number of cyclesexhibited by the data signal, computing a thickness of material removedduring one cycle of the output signal from the wavelength of the laserbeam and the index of refraction of the oxide layer of the wafer,comparing a desired thickness of material to be removed from the oxidelayer to a removed thickness comprising the product of the number ofcycles exhibited by the data signal and the thickness of materialremoved during one cycle, and terminating the CMP whenever the removedthickness equals or exceeds the desired thickness of material to beremoved. Alternately, instead of counting complete cycles, a portion ofa cycle could be counted. The procedure is almost identical except thatthe thickness of material removed is determined for the portion of thecycle, rather than for an entire cycle.

An alternate way of determining the endpoint in a CMP processing of ablank oxide wafer uses apparatus elements which measure the timerequired for the data signal to complete either a prescribed number ofcycles or a prescribed portion of one cycle, compute the thickness ofmaterial removed during the time measured, calculate a rate of removalby dividing the thickness of material removed by the time measured,ascertain a remaining removal thickness by subtracting the thickness ofmaterial removed from a desired thickness of material to be removed fromthe oxide layer, establish a remaining CMP time by dividing theremaining removal thickness by the rate of removal, and terminate theCMP process after the expiration of the remaining CMP time. In additionthis remaining CMP time can be updated after each occurrence of theaforementioned number of cycles, or portions thereof, to compensate forany in the material removal rate. In this case the procedure is almostidentical except that ascertaining the thickness of the materialinvolves first summing all the thicknesses removed in earlier iterationand subtracting this cumulative thickness from the desired thickness todetermine the remaining removal thickness figure.

However, when the wafer has an initially irregular surface topographyand is to be planarized during the CMP process, the data signal iscyclical only after the wafer surface has become smooth. In this case anendpoint to the CMP process corresponding to a determination that thewafer has been planarized is obtained by employing addition apparatuselements for detecting a cyclic variation in the data signal, andterminating the CMP whenever the detecting element detects the cyclicvariation. Preferably, the detecting element is capable of detecting acyclical variation in the data signal within at most one cycle of thebeginning of this variation.

In some circumstances, it is desirable to control the film thicknessoverlying a structure on a patterned wafer. This film thickness cannotalways be achieved through the aforementioned planarization. However,this control can still be obtained by filtering the data signal toexclude all frequencies other than that associated with the particularstructure, or group of similarly sized structures, over which a specificfilm thickness is desired. Essentially, once the signal has beenfiltered, any of the previously summarized ways of determining a CMPendpoint for a blank oxide wafer can be employed on the patterned wafer.

In addition to the just described benefits, other objectives andadvantages of the present invention will become apparent from thedetailed description which follows hereinafter when taken in conjunctionwith the drawing figures which accompany it.

DESCRIPTION OF THE DRAWINGS

The specific features, aspects, and advantages of the present inventionwill become better understood with regard to the following description,appended claims, and accompanying drawings where:

FIG. 1 is a side view of a chemical mechanical polishing (CMP) apparatustypical of the prior art.

FIG. 2 is a side view of a chemical mechanical polishing apparatus withendpoint detection constructed in accordance with the present invention.

FIGS. 3A-C are simplified cross-sectional views of respectiveembodiments of the window portion of the apparatus of FIG. 2.

FIG. 4 is a simplified cross-sectional view of a window portion of theapparatus of FIG. 2, showing components of a laser interferometergenerating a laser beam and detecting a reflected interference beam.

FIG. 5 is a simplified cross-sectional view of a blank oxide wafer beingprocessed by the apparatus of FIG. 2, schematically showing the laserbeam impinging on the wafer and reflection beams forming a resultantinterference beam.

FIG. 6 is a simplified top view of the platen of the apparatus of FIG.2, showing one possible relative arrangement between the window andsensor flag, and the sensor and laser interferometer.

FIG. 7 is a top view of the platen of the apparatus of FIG. 2, showingrelative arrangement between the window and sensor flag, and the sensorand laser, where the window is in the shape of an arc.

FIG. 8 is a block diagram of a method of piece-wise data acquisition inaccordance with the present invention.

FIGS. 9A-B are graphs showing the cyclic variation in the data signalfrom the laser interferometer over time during the thinning of a blankoxide wafer. The graph of FIG. 9A shows the integrated values of thedata signal integrated over a desired sample time, and the graph of FIG.9B shows a filtered version of the integrated values.

FIG. 10A is a block diagram of a backward-looking method of determiningthe endpoint of a CMP process to thin the oxide layer of a blank oxidewafer in accordance with the present invention.

FIG. 10B is a block diagram of a forward-looking method of determiningthe endpoint of a CMP process to thin the oxide layer of a blank oxidewafer in accordance with the present invention.

FIGS. 11A-C are simplified cross-sectional views of a patterned waferwith an irregular surface being processed by the apparatus of FIG. 2,wherein FIG. 11A shows the wafer at the beginning of the CMP process,FIG. 11B shows the wafer about midway through the process, and FIG. 11Cshows the wafer close to the point of planarization.

FIG. 12 is a block diagram of a method of determining the endpoint of aCMP process to planarize a patterned wafer with an irregular surface inaccordance with the present invention.

FIG. 13 is a graph showing variation in the data signal from the laserinterferometer over time during the planarization of a patterned wafer.

FIG. 14 is a block diagram of a method of determining the endpoint of aCMP process to control the film thickness overlying a particularly sizedstructure, or group of similarly sized structures, in accordance withthe present invention.

FIG. 15A is a simplified cross-sectional view of a wafer with a surfaceimperfection being illuminated by a narrow-diameter laser beam.

FIG. 15B is a simplified cross-sectional view of a wafer with a surfaceimperfection being illuminated by a wide-diameter laser beam.

Reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Preferred embodiments of the present invention will now be describedwith reference to the drawings.

FIG. 2 depicts a portion of a CMP apparatus modified in accordance withone embodiment of the present invention. A hole 30 is formed in theplaten 16 and the overlying platen pad 18. This hole 30 is positionedsuch that it has a view the wafer 14 held by a polishing head 12 duringa portion of the platen's rotation, regardless of the translationalposition of the head 12. A laser interferometer 32 is fixed below theplaten 16 in a position enabling a laser beam 34 projected by the laserinterferometer 32 to pass through the hole 30 in the platen 16 andstrike the surface of the overlying wafer 14 during a time when the hole30 is adjacent the wafer 14.

A detailed view of the platen hole 30 and wafer 14 (at a time when itoverlies the platen hole 30) is shown in FIGS. 3A-C. As can be seen inFIG. 3A, the platen 30 has a stepped diameter, thus forming a shoulder36. The shoulder 36 is used to contain and hold a quartz insert 38 whichfunctions as a window for the laser beam 34. The interface between theplaten 16 and the insert 38 is sealed, so that the portion of thechemical slurry 40 finding its way between the wafer 14 and insert 38cannot leak through to the bottom of the platen 16. The quartz insert 38protrudes above the top surface of the platen 16 and partially into theplaten pad 18. This protrusion of the insert 38 is intended to minimizethe gap between the top surface of the insert 38 and the surface of thewafer 14. By minimizing this gap, the amount of slurry 40 trapped in thegap is minimized. This is advantageous because the slurry 40 tends toscatter light traveling through it, thus attenuating the laser beamemitted from the laser interferometer 32. The thinner the layer ofslurry 40 between the insert 38 and the wafer 14, the less the laserbeam 34 and light reflected from the wafer, is attenuated. It isbelieved a gap of approximately 1 mm would result in acceptableattenuation values during the CMP process. However, it is preferable tomake this gap even smaller. The gap should be made as small as possiblewhile still ensuring the insert 38 does not touch the wafer 14 at anytime during the CMP process. In a tested embodiment of the presentinvention, the gap between the insert 38 and wafer 14 was set at 10 mils(250 μm) with satisfactory results.

FIG. 3B shows an alternate embodiment of the platen 16 and pad 18. Inthis embodiment, the quartz insert has been eliminated and nothrough-hole exists in the pad 18. Instead, the backing layer 20 (ifpresent) of the pad 18 has been removed in the area overlying the hole30 in the platen 16. This leaves only the polyurethane covering layer 22of the pad 18 between the wafer 14 and the bottom of the platen 16. Ithas been found that the polyurethane material used in the covering layer22 will substantially transmit the laser beam 34 from the laserinterferometer 32. Thus the portion of the covering layer 22 whichoverlies the platen hole 30 functions as a window for the laser beam 34.This alternate arrangement has significant advantages. First, becausethe pad 18 itself is used as the window, there is no appreciable gap.Therefore, very little of the slurry 40 is present to cause thedetrimental scattering of the laser beam. Another advantage of thisalternate embodiment is that pad wear becomes irrelevant. In thefirst-described embodiment of FIG. 3A, the gap between the quartz insert38 and the wafer 14 was made as small as possible. However, as the pad18 wears, this gap tend to become even smaller. Eventually, the wearcould become so great that the top surface of the insert 38 would touchthe wafer 14 and damage it. Since the pad 18 is used as the window inthe alternate embodiment of FIG. 3B, and is designed to be in contactwith the wafer 14, there are no detrimental effects due to theaforementioned wearing of the pad 18. It is noted that tests using boththe open-cell and grooved surface types of pads have shown that thelaser beam is less attenuated with the grooved surface pad. Accordingly,it is preferable that this type of pad be employed.

Although the polyurethane material used in the covering layer of the padis substantially transmissive to the laser beam, it does contain certainadditives which inhibit its transmissiveness. This problem is eliminatedin the embodiment of the invention depicted in FIG. 3C. In thisembodiment, the typical pad material in the region overlying the platenhole 30 has been replaced with a solid polyurethane plug 42. This plug42, which functions as the window for the laser beam, is made of apolyurethane material which lacks the grooving (or open-cell structure)of the surrounding pad material, and is devoid of the additives whichinhibit transmissiveness. Accordingly, the attenuation of the laser beam34 through the plug 42 is minimized. Preferably, the plug 42 isintegrally molded into the pad 18.

In operation, a CMP apparatus in accordance with the present inventionuses the laser beam from the laser interferometer to determine theamount material removed from the surface of the wafer, or to determinewhen the surface has become planarized. The beginning of this processwill be explained in reference to FIG. 4. It is noted that a laser andcollimator 44, beam splitter 46, and detector 48 are depicted aselements of the laser interferometer 32. This is done to facilitate theaforementioned explanation of the operation of the CMP apparatus. Inaddition, the embodiment of FIG. 3A employing the quartz insert 38 as awindow is shown for convenience. Of course, the depicted configurationis just one possible arrangement, others can be employed. For instance,any of the aforementioned window arrangements could be employed, andalternate embodiments of the laser interferometer 32 are possible. Onealternate interferometer arrangement would use a laser to produce a beamwhich is incident on the surface of the wafer at an angle. In thisembodiment, a detector would be positioned at a point where lightreflecting from the wafer would impinge upon it. No beam splitter wouldbe required in this alternate embodiment.

As illustrated in FIG. 4, the laser and collimator 44 generate acollimated laser beam 34 which is incident on the lower portion of thebeam splitter 46. A portion of the beam 34 propagates through the beamsplitter 46 and the quartz insert 38. Once this portion of beam 34leaves the upper end of the insert 38, it propagates through the slurry40, and impinges on the surface of the wafer 14. The wafer 14, as shownin detail in FIG. 5 has a substrate 50 made of silicon and an overlyingoxide layer 52 (i.e. SiO₂).

The portion of the beam 34 which impinges on the wafer 14 will bepartially reflected at the surface of the oxide layer 52 to form a firstreflected beam 54. However, a portion of the light will also betransmitted through the oxide layer 52 to form a transmitted beam 56which impinges on the underlying substrate 50. At least some of thelight from the transmitted beam 56 reaching substrate 50 will bereflected back through the oxide layer 52 to form a send reflected beam58. The first and second reflected beams 54, 58 interfere with eachother constructively or destructively depending on their phaserelationship, to form a resultant beam 60, where the phase relationshipis primarily a function of the thickness of the oxide layer 52.

Although, the above-described embodiment employs a silicon substratewith a single oxide layer, those skill in the art will recognize theinterference process would also occur with other substrates and otheroxide layers. The key is that the oxide layer partially reflects andpartially transmits, and the substrate at least partially reflect, theimpinging beam. In addition, the interference process may also beapplicable to wafers with multiple layers overlying the substrate.Again, if each layer is partially reflective and partially transmissive,a resultant interference beam will be created, although it will be acombination of the reflected beams from all the layer and the substrate.

Referring again to FIG. 4, it can be seen the resultant beam 60representing the combination of the first and second reflected beams 54,58 (FIG. 5) propagates back through the slurry 40 and the insert 38, tothe upper portion of the beam splitter 46. The beam splitter 46 divertsa portion of the resultant beam 60 towards the detector 48.

The platen 16 will typically be rotating during the CMP process.Therefore, the platen hole 30 will only have a view of the wafer 14during part of its rotation. Accordingly, the detection signal from thelaser interferometer 32 should only be sampled when the wafer 14 isimpinged by the laser beam 34. It is important that the detection signalnot be sampled when the laser beam 34 is partially transmitted throughthe hole 30, as when a portion is blocked by the bottom of the platen 16at the hole's edge, because this will cause considerate noise in thesignal. To prevent this from happening the position sensor apparatus hasbeen incorporated. Any well known proximity sensor could be used, suchas Hall effect, eddy current, optical interrupter, and acoustic sensor,although an optical interrupter type sensor was used in the testedembodiments of the invention and will be shown in the figures thatfollow. An apparatus accordingly to the present invention forsynchronizing the laser interferometer 32 is shown in FIG. 6, with anoptical interrupter type sensor 62 (e.g. LED/photodiode pair) mounted ona fixed point on the chassis of the CMP device such that it has a viewof the peripheral edge of the platen 16. This type of sensor 62 isactivated when an optical beam it generates is interrupted. A positionsensor flag 64 is attached to the periphery of the platen 16. The pointof attachment and length of the flag 64 is made such that it interruptsthe sensor's optical signal only when the laser beam 34 from the laserinterferometer 32 is completely transmitted through thepreviously-described window structure 66. For example, as shown in FIG.6, the sensor 62 could be mounted diametrically opposite the laserinterferometer 32 in relation to the center of the platen 16. The flag64 would be attached to the platen 16 in a position diametricallyopposite the window structure 66. The length of the flag 64 would beapproximately defined by the dotted lines 68, although, the exact lengthof the flag 64 would be fine tuned to ensure the laser beam iscompletely unblocked by the platen 16 during the entire time the flag 64is sensed by the sensor 62. This fine tuning would compensate for anyposition sensor noise or inaccuracy, the responsiveness of the laserinterferometer 32, etc. Once the sensor 62 has been activated, a signalis generated which is used to determine when the detector signal fromthe interferometer 32 is to be sampled.

Data acquisition systems capable of using the position sensor signal tosample the laser interferometer signal during those times when the waferis visible to the laser beam, are well known in the art and do not forma novel part of the present invention. Accordingly, a detaileddescription will not be given herein. However some considerations shouldbe taken into account in choosing an appropriate system. For example, itis preferred that the signal from the interferometer be integrated overa period of time. This integration improves the signal-to-noise ratio byaveraging the high frequency noise over the integration period. Thisnoise has various causes, such as vibration from the rotation of theplaten and wafer, and variations in the surface of the wafer due tounequal planarization. In the apparatus described above the diameter ofthe quartz window, and the speed of rotation of the platen, willdetermine how long a period of time is available during any one rotationof the platen to integrate the signal. However, under somecircumstances, this available time may not be adequate. For instance, anacceptable signal-to-noise ratio might require a longer integrationtime, or the interface circuitry employed in a chosen data acquisitionsystem may require a minimum integration time which exceeds that whichis available in one pass.

One solution to this problem is to extend the platen hole along thedirection of rotation of the platen. In other words, the windowstructure 66′ (i.e. insert, pad, or plug) would take on the shape of anarc, as shown in FIG. 7. Of course, the flag 64′ is expanded toaccommodate the longer window structure 66′. Alternately, the windowcould remain the same, but the laser interferometer would be mounted tothe rotating platen directly below the window. In this case, the CMPapparatus would have to be modified to accommodate the interferometerbelow the platen, and provisions would have to be made route thedetector signal from the interferometer. However, the net result ofeither method would be to lengthen the data acquisition time for eachrevolution of the platen.

Although lengthening the platen hole and window is advantageous, it doessomewhat reduce the surface area of the platen pad. Therefore, the rateof planarization is decreased in the areas of the disk which overlie thewindow during a portion of the platen's rotation. In addition, thelength of the platen hole and window must not extend beyond the edges ofthe wafer, and the data sampling must not be done when the window isbeyond the edge of the wafer, regardless of the wafer's translationalposition. Therefore, the length of the expanded platen hole and window,or the time which the platen-mounted interferometer can be sampled, islimited by any translational movement of the polishing head.

Accordingly, a more preferred method of obtaining adequate dataacquisition integration time is to collect the data over more than onerevolution of the platen. In reference to FIG. 8, during step 102, thelaser interferometer signal is sampled during the available dataacquisition time in each rotation of the platen. Next, in steps 104 and106, each sampled signal is integrated over the aforementioned dataacquisition time, and the integrated values are stored. Then, in steps108 and 110, a cumulative sample time is computed after each completerevolution of the platen and compared to a desired minimum sample time.Of course, this would constitute only one sample time if only one samplehas been taken. If the cumulative sample time equals or exceeds thedesired minimum sample time, then the stored integrated values aretransferred and summed, as shown in step 112. If not, the process ofsampling, integrating, storing, computing the cumulative sample time,and comparing it to the desired minimum sample time continues. In afinal step 114, the summed integrated values created each time thestored integrated values are transferred and summed, are output as adata signal. The just-described data collection method can beimplemented in a number of well known ways, employing either logiccircuits or software algorithms. As these methods are well known, anydetailed description would be redundant and so has been omitted. It isnoted that the method of piece-wise data collection provides a solutionto the problem of meeting a desired minimum sample time no matter whatthe diameter of the window or the speed of platen rotation. In fact, ifthe process is tied to the position sensor apparatus, the platenrotation speed could e varied and reliable data would still be obtained.Only the number of platen revolutions required to obtain the necessarydata would change.

The aforementioned first and second reflected beams which formed theresultant beam 60, as shown in FIGS. 4 and 5, cause interference to beseen at the detector 48. If the first and second beams are in phase witheach other, they cause a maxima on detector 48. Whereas, if the beamsare 180 degrees out of phase, they cause a minima on the detector 48.Any other phase relationship between the reflected beams will result inan interference signal between the maxima and minima being seen by thedetector 48. The result is a signal output from the detector 48 thatcyclically varies with the thickness of the oxide layer 52, as it isreduced during the CMP process. In fact, it has been observed that thesignal output from the detector 48 will vary in a sinusoidal-likemanner, as shown in the graphs of FIGS. 9A-B. The graph of FIG. 9A showsthe integrated amplitude of the detector signal (y-axis) over eachsample period versus time (x-axis). This data was obtained by monitoringthe laser interferometer output of the apparatus of FIG. 4, whileperforming the CMP procedure on a wafer having a smooth oxide layeroverlying a silicon substrate (i.e. a blank oxide wafer). The graph ofFIG. 9B represents a filtered version of the data from the graph of FIG.9A. This filtered version shows the cyclical variation in theinterferometer output signal quite clearly. It should be noted that theperiod of the interference signal is controlled by the rate at whichmaterial is removed from the oxide layer during the CMP process. Thus,factors such as the downward force placed on the wafer against theplaten pad, and the relative velocity between the platen and the waferdetermine the period. During each period of the output signal plotted inFIGS. 9A-B, a certain thickness of the oxide layer is removed. Thethickness removed is proportional to the wavelength of the laser beamand the index of refraction of the oxide layer. Specifically the amountof thickness removed per period is approximately λ/2n, where λ is thefreespace wavelength of the laser beam and n is the index of refractionof the oxide layer. Thus, it is possible to determine how much of theoxide layer is removed, in-situ, during the CMP process using the methodillustrated in FIG. 10A. First, in step 202, the number of cyclesexhibited by the data signal are counted. Next, in step 204, thethickness of the material removed during one cycle of the output signalis computed from the wavelength of the laser beam and the index ofrefraction of the oxide layer of the wafer. Then, the desired thicknessof material to be removed from the oxide layer is compared to the actualthickness removed, in step 206. The actual thickness removed equals theproduct of the number of cycles exhibited by the data signal and thethickness of material removed during one cycle. In the final step 208,the CMP process is terminated whenever the removed thickness equals orexceeds the desired thickness of material to be removed.

Alternately, less than an entire cycle might be used to determine theamount of material removed. In this way any excess material removed overthe desired amount can be minimized. As shown in the bracketed portionsof the step 202 in FIG. 10A, the number of occurrences of a prescribedportion of a cycle are counted in each iteration. For example, eachoccurrence of a maxima (i.e. peak) and minima (i.e. valley), or viceversa, would constitute the prescribed portion of the cycle. Thisparticular portion of the cycle is convenient as maxima and minima arereadily detectable via well know signal processing methods. Next, instep 204, after determining how much material is removed during a cycle,this thickness is multiplied by the fraction of a cycle that theaforementioned prescribed portion represents. For example in the case ofcounting the occurrence of a maxima and minima, which representsone-half of a cycle, the computed one-cycle thickness would bemultiplied by one-half to obtain the thickness of the oxide layerremoved during the prescribed portion of the cycle. The remaining stepsin the method remain unchanged. The net result of this alternateapproach is that the CMP process can be terminated after the occurrenceof a portion of the cycle. Accordingly, any excess material removedwill, in most cases, be less than it would have been if a full cyclewhere is as the basis for determining the amount of material removed.

The just-described methods look back from the end of a cycle, or portionthereof, to determine if the desired amount of material has beenremoved. However, as inferred above, the amount of material removedmight exceed the desired amount. In some applications, this excessremoval of material might be unacceptable. In these cases, an alternatemethod can be employed which looks forward and anticipates how muchmaterial will be removed over an upcoming period of time and terminatesthe procedure when the desired thickness is anticipated to have beenremoved. A preferred embodiment of this alternate method is illustratedin FIG. 10B. As can be seen, the first step 302 involves measuring thetime between the first occurrence of a maxima and minima, or vice versa,in the detector signal (although an entire cycle or any portion thereofcould have been employed). Next, in step 304, the amount of materialremoved during that portion of the cycle is determined via thepreviously described methods. A removal rate is then calculated bydividing the amount of material removed by the measured time, as shownin step 306. This constitutes the rate at which material was removed inthe preceding portion of the cycle. In the next step 308, the thicknessof the material removed as calculated in step 304 is subtracted from thedesired thickness to be removed to determine a remaining removalthickness. Then, in step 310, this remaining removal thickness isdivided by the aforementioned removal rate to determine how much longerthe CMP process is to be continued before it termination.

It must be noted, however, that the period of the detector signal, andso the removal rate, will typically vary as the CMP process progresses.Therefore, the above-described method is repeated to compensate. Inother words, once a remaining time has been calculated, the process isrepeated for each occurrence of a maxima and minima, or vice versa.Accordingly, the time between the next occurring maxima and minima ismeasured, the thickness of material removed during the portion of thecycle represented by this occurrence of the maxima and minima (i.e.one-half) is divided by the measured time, and the removal rate iscalculated, just as in the first iteration of the method. However, inthe next step 308, as shown in brackets, the total amount of materialremoved during all the previous iterations is determined before beingsubtracted from the desired thickness. The rest of the method remainsthe same in that the remaining thickness to be removed is divided by thenewly calculated removal rate to determine the remaining CMP processtime. In this way the remaining process time is recalculated after eachoccurrence of the prescribed portion of a cycle of the detector signal.This process continues until the remaining CMP process time will expirebefore the next iteration can begin. At that point the CMP process isterminated, as seen in step 312. Typically, the thickness to be removedwill not be accomplished in the first one-half cycle of the detectorsignal, and any variation in the removal rate after being calculated forthe preceding one-half cycle will be small. Accordingly, it is believethis forward-looking method will provide a very accurate way of removingjust the desired thickness from the wafer.

While the just-described monitoring procedure works well for thesmooth-surfaced blank oxide wafers being thinned, it has been found thatthe procedure cannot be successfully used to planarize most patternedwafers where the surface topography is highly irregular. The reason forthis is that a typical patterned wafer contains dies which exhibit awide variety of differently sized surface features. These differentlysized surface features tend to polish at different rates. For example, asmaller surface feature located relatively far from other features tendsto be reduced faster than other larger features. FIGS. 11A-C exemplify aset of surface features 72, 74, 76 of the oxide layer 52 associated withunderlying structures 78, 80, 82, that might be found on a typicalpatterned wafer 14, and the changes they undergo during the CMP process.Feature 72 is a relatively small feature, feature 74 is a medium sizedfeature, and feature 76 is a relatively large feature. FIG. 11A showsthe features 72, 74, 76 before polishing, FIG. 11B shows the features72, 74, 76 about midway through the polishing process, and FIG. 11Cshows the features 72, 74, 76 towards the end of the polishing process.In FIG. 11A, the smaller feature 72 will be reduced at a faster ratethan either the medium or large features 74, 76. In addition, the mediumfeature 74 will be reduced at a faster rate than the large feature 76.The rate at which the features 72, 74, 76 are reduced also decreases asthe polishing process progresses. For example, the smaller feature 72will initially have a high rate of reduction, but this rate will dropoff during the polishing process. Accordingly, FIG. 11B shows the heightof the features 72, 74, 76 starting to even out, and FIG. 11C shows theheight of the features 72, 74, 76 essentially even. Since thedifferently sized features are reduced at different rates and theserates are changing, the interference signal produced from each featurewill have a different phase and frequency. Accordingly, the resultantinterference signal, which is partially made up of all the individualreflections from each of the features 72, 74, 76, will fluctuate in aseemingly random fashion, rather than the previously described periodicsinusoidal signal.

However, as alluded to above, the polishing rates of the features 72,74, 76 tend to converge closer to the point of planarization. Therefore,the difference in phase and frequency between the interference beamsproduced by the features 72, 74, 76 tend to approach zero. This resultsin the resultant interference signal becoming recognizable as a periodicsinusoidal wave form. Therefore it is possible to determine when thesurface of a patterned wafer has become planarized by detecting when asinusoidal interference signal begins. This method is illustrated inFIG. 12. First, in step 402, a search is made for the aforementionedsinusoidal variation in the interferometer signal. When the sinusoidalvariation is discovered, the CMP procedure is terminated, as shown instep 404.

FIG. 13 is a graph plotting the amplitude of the detector signal overtime for a patterned wafer undergoing a CMP procedure. The sampled dataused to construct this graph was held at its previous integrated valueuntil the next value was reported, thus explaining the squared-off peakvalues shown. A close inspection shows that a discernible sinusoidalcycle begins to emerge at approximately 250 seconds. This coincides withthe point where the patterned wafer first became planarized. Of course,in real-time monitoring of the interferometer's output signal, it wouldbe impossible to know exactly when the cycling begins. Rather, at leastsome portion of the cycle must have occurred before it can be certainthat the cycling has begun. Preferably, no more than one cycle isallowed to pass before the CMP procedure is terminated. A one-cyclelimit is a practical choice because it provides a high confidence thatthe cycling has actually begun, rather than the signal simplyrepresenting variations in the noise caused by the polishing of thedifferently sized features on the surface of the wafer. In addition, theone-cycle limit ensures only a small amount of material is removed fromthe surface of the wafer after it becomes planarized. It has been foundthat the degree of planarization is essentially the same after twocycles, as it was after one. Thus, allowing the CMP procedure tocontinue would only serve to remove more material from the surface ofthe wafer. Even though one cycle is preferred in the case where the CMPprocess is to be terminated once the patterned wafer becomes planarized,it is not intended that the present invention be limited to thattimeframe. If the signal is particularly strong, it might be possible toobtain the same level of confidence after only a portion of a cycle.Alternately, if the signal is particularly weak, it may take more thanone cycle to obtain the necessary confidence. The choice will depend onthe characteristics of the system used. For instance, the size of thegap between the quartz window and the surface of the wafer will have aneffect on signal strength, and so the decision on how many cycles towait before terminating the CMP process.

The actual determination as to when the output signal from the laserinterferometer is actually cycling, and so indicating that the surfaceof the wafer has been planarized can be done in a variety of ways. Forexample, the signal could be digitally processed and an algorithmemployed to make the aforementioned determination. Such a method isdisclosed in U.S. Pat. No. 5,097,430, where the slope of the signal isused to make the determination. In addition, various well known curvefitting algorithms are available. These methods would essentially beused to compare the interferometer signal to a sinusoidal curve. When amatch occurs within some predetermined tolerance, it is determined thatthe cycling has begun.

Some semiconductor applications require that the thickness of thematerial overlying a structure formed on a die of a patterned wafer(i.e. the film thickness) be at a certain depth, and that this filmthickness be repeatable from die to die, and from wafer to wafer. Thepreviously described methods for planarizing a typical patterned waferwill not necessarily produce this desired repeatable film thickness. Thepurpose of the planarization methods is to create a smooth and flatsurface, not to produce a particular film thickness. Accordingly, if itis desirable to control the film thickness over a specific structure, orgroup of similarly sized structures, an alternate method must beemployed. This alternate method is described below.

As alluded to previously, each differently sized surface featureresulting from a layer of oxide being formed over a patterned structureon a die tends to produce a reflected interference signal with a uniquefrequency and phase. It is only close to the point of planarization thatthe frequency and phase of each differently sized feature converges.Prior to this convergence the unique frequency and phase of theinterference signals caused by the various differently sized featurescombine to produce a detector signal that seems to vary randomly.However, it is possible to process this signal to eliminate theinterference signal contributions of all the features being polished atdifferent rates, except a particularly sized feature, or group ofsimilarly sized features. Once the interference signal associated withthe particularly sized feature, or group of features, has been isolated,the methods discussed in association with the removal of material from ablank oxide disk are employed to remove just the amount of materialnecessary to obtain the desired film thickness.

Of course, the frequency of the interference signal component caused bythe feature of interest must be determined prior to the signalprocessing. It is believed this frequency can be easily determined byperforming a CMP process on a test specimen which includes diesexclusively patterned with structures corresponding to the structurewhich is to have a particular overlying film thickness. The detectorsignal produced during this CMP process is analyzed via well knownmethods to determine the unique frequency of the interference signalcaused by the surface features associated with the aforementionedstructures.

The specific steps necessary to perform the above-described method ofcontrolling the film thickness over a specific structure, or group ofsimilarly sized structures on a die, in situ, during the CMP processingof a wafer, will now be described in reference to FIG. 14. In step 502,the detector signal is filtered to pass only the component of the signalhaving the predetermined frequency associated with the structure ofinterest. This step is accomplished using well known band-pass filteringtechniques. Next in step 504 a measurement is made of the time betweenthe first occurrence of a maxima and minima, or vice versa, in thedetector signal (although an entire cycle or any portion thereof couldhave been employed). The amount of material removed during that portionof the cycle (i.e. one-half cycle) is determined in step 506 viapreviously described methods. Then, a removal rate is then calculated bydividing the amount of material removed by the measured time, as shownin step 508. This constitutes the rate at which material was removed inthe preceding portion of the cycle. In the next step 510, the thicknessof the material removed as calculated in step 506 is subtracted from thedesired thickness to be removed (i.e. the thickness which when removedwill result in the desired film thickness overlying the structure ofinterest), to determine a remaining removal thickness. Then, thisremaining removal thickness is divided by the aforementioned removalrate to determine how much longer the CMP process is to be continuedbefore it termination, in step 512. Once a remaining time has beencalculated, the process is repeated for each occurrence of a maxima andminima, or vice versa. Accordingly, the time between the next occurringmaxima and minima is measured, the thickness of material removed duringthe portion of the cycle represented by this occurrence of the maximaand minima (i.e. one-half) is divided by the measured time, and theremoval rate is calculated, just as in the first iteration of themethod. However, in the next step 510, as shown in brackets, the totalamount of material removed during all the previous iterations isdetermined before being subtracted from the desired thickness. The restof the method remains the same in that the remaining thickness to beremoved is divided by the newly calculated removal rate to determine theremaining CMP process time. This process is repeated until the remainingtime expires before the next iteration can begin. At that point, the CMPprocess is terminated, as seen in step 514.

It is noted that although the method for controlling film thicknessdescribed above utilizes the method for determining the CMP processendpoint illustrated in FIG. 10B, any of the other endpointdetermination methods described herein could also be employed, ifdesired.

It is further noted that the beam diameter (i.e. spot) and wavelength ofthe laser beam generated by the laser interferometer can beadvantageously manipulated. As shown in FIGS. 15A and 15B, a narrow beam84, such as one focused to the smallest spot possible for the wavelengthemployed, covers a smaller area of the surface of the wafer 14 than awider, less focused beam 86. This narrow beam 84 is more susceptible toscattering (i.e. beam 88) due to surface irregularities 90, than thewider beam 86, since the wider beam 86 spreads out over more of thesurface area of the wafer 14, and encompasses more of the surfaceirregularities 90. Therefore, a wider beam 86 would have an integratingeffect and would be less susceptible to extreme variations in thereflected interference signal, as it travels across the surface of thewafer 14. Accordingly, a wider beam 86 is preferred for this reason. Thelaser beam with can be widened using well known optical devices.

It must also be pointed out that the wider beam will reduce theavailable data acquisition time per platen revolution since the time inwhich the beam is completely contained within the boundaries of thewindow is less than it would be with a narrower beam. However, with thepreviously described methods of data acquisition, this should notpresent a significant problem. In addition, since the wider beam alsospreads the light energy out over a larger area than a narrower beam,the intensity of the reflections will be lessen somewhat. This drawbackcan be remedied by increasing the power of the laser beam from the laserinterferometer so that the loss in intensity of the reflected beams isnot a factor in detection.

As for the wavelength of the laser beam, it is feasible to employ awavelength anywhere from the far infrared to ultraviolet. However, it ispreferred that a beam in the red light range be used. The reason forthis preference is two-fold. First, shorter wavelengths result in anincrease in the amount of scattering caused by the chemical slurrybecause this scattering is proportional to the 4th power of thefrequency of the laser beam. Therefore, the longer the wavelength, theless the scattering. However, longer wavelengths also result in more ofthe oxide layer being removed per period of the interference signal,because the amount of material removed per period equals approximatelyλ/2n. Therefore, the shorter the wavelength, the less material removedin one period. It is desirable to remove as little of the material aspossible during each period so that the possibility of any excessmaterial being removed is minimized. For example, in a system employingthe previously described method by which the number of cycles, or aportion thereof, are counted to determine the thickness of the oxidelayer removed, any excess material removed over the desired amount wouldbe minimized if the amount of material removed during each cycle, orportion thereof, is as small as possible.

It is believed these two competing factors in the choice of wavelengthare optimally balance if a red light laser beam is chosen. Red lightoffers an acceptable degree of scattering while not resulting in anunmanageable amount of material being removed per cycle.

While the invention has been described in detail by reference to thepreferred embodiment described above, it is understood that variationsand modifications thereof may be made without departing from the truespirit and scope of the invention.

Wherefor, what is claimed is:
 1. An apparatus for chemical mechanicalpolishing of a wafer, comprising: a rotatable platen to hold a polishingpad, the platen having a hole therein, wherein the hole extends entirelythrough the platen; a polishing head for holding the wafer against thepolishing pad; an optical monitoring system including a light source todirect a light beam through the aperture toward the wafer from a side ofthe wafer contacting the polishing pad and a detector to receivereflections of the light beam from the wafer, wherein the light sourceis stationary, the detector is stationary and the hole intermittentlyprovides a pathway for the light beam to impinge on the wafer; and aposition sensor for sensing when the hole is adjacent the wafer suchthat the light beam generated by the light source can pass through thehole and impinge on the wafer, wherein the position sensor includes aflag and an optical interrupter-type sensor and the flag is attachedalong a portion of a periphery of the platen and the sensor is mountedto a chassis at the periphery of the platen.
 2. The apparatus of claim1, further comprising a light-transmitting insert mounted in the hole.3. The apparatus of claim 1, further comprising the polishing pad andwherein the polishing pad includes a light-transmitting portion adjacentthe hole.
 4. The apparatus of claim 1, wherein the light sourcecomprises a laser and the light beam comprises a laser beam.
 5. Theapparatus of claim 4, wherein the laser beam has a wavelength inapproximately the red light range.
 6. The apparatus of claim 1, whereinthe sensor is configured to produce and detect an optical beam and theflag is positioned to interrupt the optical beam pass such that theoptical beam is interrupted by the flag when the light beam can passthrough the hole and impinge on the wafer.
 7. The apparatus of claim 1,wherein the optical monitoring system further comprises data acquisitionmeans connected to the detector and position sensor for sampling asignal from the detector while the position sensor senses that the holeis adjacent the wafer and outputting a data signal representing thesampled signal.
 8. The apparatus of claim 7, wherein the dataacquisition means includes means for integrating the sampled detectionsignal over a predetermined period of time.
 9. The apparatus of claim 8,wherein the predetermined period of time is a time during a singlerotation of the platen.
 10. The apparatus of claim 8, wherein thepredetermined period of time is a time during a single rotation of theplaten during which the position sensor senses that the hole is adjacentthe wafer.
 11. The apparatus of claim 1, further comprising an endpointdetector to determine a removed thickness and terminating polishing whenthe removed thickness equals or exceeds the desired thickness ofmaterial to be removed.
 12. The apparatus of claim 1, wherein a datasignal output by optical monitoring system is cyclical, and wherein theendpoint detector is configured to count a number of cycles exhibited bythe data signal and compute a thickness of material removed from thenumber of cycles, the wavelength of the light beam and the index ofrefraction of an oxide layer of the wafer.